The present invention relates to a novel method for writing and erasing. More particularly, the present invention relates to a colorless dye/developer system ink which can give deep, clear handwriting, and to an eraser which can easily erase the handwriting written in the ink. Further, the present invention relates to a combination of a writing utensil employing an ink and an erasing utensil employing the eraser.
Heretofore, there have been developed various methods for erasing handwriting written in ink. For example, there is a method (1) in which handwriting written in ink employing an easily bleachable dye is erased by means of a bleaching agent, and a method (2) in which a colorant is incorporated in a rubbery high-molecular weight material, the resulting material is dissolved in a solvent to produce ink, and handwriting written in this ink is erased by a rubber eraser. However, these methods have the following disadvantages. In method (1), traces of erased ink marks are liable to yellow and the stability of the bleaching agent with time is poor. In method (2), since the viscosity of the ink is highly increased, the use of a pressurized ball point pen structure becomes necessary and, further, the erasing performance is not thought to be fully satisfactory.
Heretofore, there has been proposed a writing method utilizing a reversible reaction in which color development is caused by electron transfer between a developer having a phenolic hydroxyl group and a colorless electron-donating organic compound (a colorless dye) which develops a color by the action of said developer, and this color development caused by the electron transfer is inhibited by a desensitizer consisting of a polar compound, whereby the colored dye is made colorless. For example, there is disclosed in Japanese patent publication No. 21649/1973 a writing material consisting of a dye (a colorless dye) having a developing component structure in the molecule, a compound (a developer) having an acidic hydroxyl group in the molecule and a polar solvent (a desensitizer). This writing material is intended to provide a special-purpose ink which is colorless just before writing and which gives visible handwriting by evaporation of the polar solvent after writing. Since the handwriting can not be recognized at the time it is written, this writing material is unsuitable as ink for usual writing utensils. Further, there is disclosed in Japanese patent publication No. 48085/1976 a colorant for writing utensils, which consists of an electron-donating organic compound (a colorless dye), a compound having a phenolic hydroxyl group (a developer) and a nonvolatile compound (a desensitizer) desensitizing a color developing reaction between said two compounds. Handwriting written in such a colorant is colored immediately after it is written. However, when the developer in said handwriting is evaporated by heating or washed off by water, the handwriting is made colorless or discolored. Such an erasing technique is not simple as compared with means employing a rubber eraser or an erasing liquid. Thus, this colorant is unsuitable for use as erasable ink for usual writing utensils. Further, there can not be obtained stable fast handwriting which does not suffer discoloration nor decoloration with time as in the usual ink.